1. Field of the Invention
The present invention relates to a variable capacitor and, in particular, to a variable capacitor having a stator and a rotor that is rotatably supported relative to the stator, in which rotation of the rotor changes the area of overlap of a stator electrode and a rotor electrode so as to change the capacitance.
2. Description of the Related Art
Variable capacitors in which rotation of a rotor having a rotor electrode changes the area of overlap of a stator electrode in a stator and the rotor electrode to change their capacitance are disclosed in Japanese Unexamined Patent Application Publication No.10-233338 (hereinafter referred to as Patent Document 1) and U.S. Pat. No. 4,112,480 (hereinafter referred to as Patent Document 2).
FIG. 8 shows the equivalent circuit of the variable capacitor disclosed in Patent Document 1. Rotation of a single rotor changes two capacitances C11 and C12. The variable capacitor is designed so that each capacitance C11 or C12 has a minimum value and a maximum value during one rotation of the rotor.
Unfortunately, in such a capacitor, the capacitances C11 and C12, which are connected in series, disadvantageously increase a resistance (ESR) resulting from a resistive component induced in the stator electrode.
On the other hand, Patent Document 2 discloses a variable capacitor in which layers of stator electrodes are disposed in a dielectric substrate and, as shown in the equivalent circuit in FIG. 9, one fixed capacitance C13 is connected in parallel to one variable capacitance C14. This variable capacitor can reduce a resistance (ESR) resulting from a resistive component induced in the stator electrode.
Unfortunately, in this variable capacitor, the value of the capacitance C13 is fixed, and therefore, the fixed capacitance C13 determines the lowest value of the overall capacitance. As a result, the capacitance of the variable capacitor cannot be adjustable to an extremely small value or zero. That is, the variable capacitor has a low self-oscillation frequency, thus making it difficult to operate in a high frequency range.